1. Field of the Invention
The present invention relates to a keyswitch assembly for use on a keyboard to be used as the input device of a word processor or a personal computer.
2. Description of Related Art
A prior art keyswitch assembly, such as disclosed in U.S. Pat. Nos. 4,580,022 and 4,560,845, for use on a keyboard has a key provided with a stem, a base plate provided with a guide member having a hole receiving the stem of the key to guide the key for vertical movement, a rubber spring provided with a contact on the inner surface of the upper wall thereof and disposed below the stem, and a switching device, such as a membrane switch consisting of electric contacts formed on a flexible sheet.
In a keyswitch assembly disclosed in U.S. Pat. No. 4,580,022, a key of a relatively large size as compared with corresponding electrical contacts, such as a space key, is supported on a key support mechanism formed by pivotally joining a pair of support levers in the middle portions thereof in a scissors-like form. The key is placed on a base plate to enable the key to move downward in a level position without being tilted when depressed.
Recent progressive reduction in the thickness of keyboards requires reduction in the thickness of the key. However, on the other hand, the stroke of the key must be sufficiently large to facilitate keystroke operation and to ensure reliable keystroke.
However, if the thickness of the keyboard provided with the foregoing prior art keyswitch assembly having the key provided with the stem projecting from the lower surface thereof and the guide member having the hole receiving the stem of the key to guide the key for vertical movement is desired to be reduced, the length of a sliding portion of the stem in sliding engagement with the guide member must be reduced. If the length of the sliding portion of the stem is reduced, the key is liable to tilt relative to the guide member and, consequently, the stem is liable to slide awkwardly in the guide member when the key is depressed. Therefore, the key is unable to be operated smoothly and lightly. If the length of the sliding portion of the stem in engagement with the guide member is increased to ensure smooth movement of the key, the stroke of the key is reduced. Thus, the effort of reducing the thickness of the keyboard and the effort of securing a sufficiently large key stoke are contradictory.
Further, in order to ensure accurate operation by the operator, satisfactory tactile sensation upon depression of the keys is necessary. In prior art switching members which utilize flexible membrane switches, the shape of the switching device is designed on the assumption that the switch is depressed linearly, and upon depression, the switching device is designed to buckle. Such devices are formed of membranes having a uniform thickness. However, a membrane of uniform thickness will not provide satisfactory tactile feedback since there is not a sufficiently sharp change in the resistance of the key against depression at the moment the electrical contact is closed. Further, if the key is not pressed perfectly linearly downward, the pressure will be inefficiently transferred to the switching device and the switching device will buckle indefinitely.